Have you ever heard the old saying "Drought Begets Drought..."? There is actually truth to the saying and it can be explained using atmospheric soundings. Soundings are a tool that meteorologists/forecasters use to get a handle on how temperature, moisture, and winds are distributed through the depth of the atmosphere. This image below is complex, however step by step instructions of how to interpret the image are listed below. Once an understanding of what the sounding represents is gained, it will be much easier to understand why drought conditions tend to lead toward prolonged periods of dry weather.
For your convenience, the image above is color coded to help with understanding the different features. The cloud and rain drops on the image were added and are not part of a normal sounding.
The earth's surface is near the bottom of the image, with increaing height indicated by the white arrow pointing to 10,000 feet and above.
Temperature - The white line that is labeled by the word Temperature represents how warm or cool the air is through the entire depth of the atmosphere. In general, air is warmest at the surface and tends to cool with height. (An example of this is snow on the Rocky Mountains well into Spring and Summer!) Notice how the line leans toward the left(cooler) as it increases through 10,000 feet and beyond.
Dew Point - The white line that is labeled by the word Dew Point is to the left of the temperature line. This line helps to measure how dry or moist the air is throughout the depth of the atmosphere. Areas where the temperature and dew point lines are closer together represent where the air is more saturated or more moist. The farther apart the lines are, the more dry the air becomes. Where the lines are closest together are where clouds are most likely to form. A cloud has been placed at the location where the lines are closest together on this sounding, however the lines generally need to be within 5 degrees Celsius or 9 degrees Fahrenheit to be saturated enough to produce a cloud.
So how does a drought tend to prolong precipitation events from occurring? There are several reasons. First of all, drier soil conditions lead to warmer temperatures than moist soil because not as much of the sun's energy is lost in trying to evaporate water. In other words, more energy goes into heating and less into evaporating. Also, if there is less rainfall for plants, trees, grass, etc. to absorb and less water in rivers, lakes, etc. to evaporate there is less overall moisture to evapotranspirate into the lower atmosphere. This effectively lowers the amount of moisture in the lower atmosphere.
So, looking back at our sounding, imagine the temperature line moving right (warmer) due to better heating. Now imagine the dew point line moving left (drier) because plants and lakes are not emitting as much moisture into the lower atmosphere. The end result is that the two lines are farther apart and the entire lower atmosphere is now even drier!
How does this impact precipitation events? Imagine the cloud precipitating rainfall from above on our example sounding. The rain is falling from an area where there is much more moisture, however as it approaches the earth's surface it is falling into a much drier environment. Raindrops that fall into a drier environment will quickly evaporate. It takes a lot of rain to fall before the lower atmosphere slowly saturates and is able to produce rain without it evaporating. Therefore, in a drought situation, it takes stronger preciptiation events for it to actually rain.